Project Summary: Stem cells replenish tissues and organs over an organism?s lifetime and can repair damage after injury. With their special capacities for self-renewal and differentiation, stem cells promise to revolutionize medicine. To develop better and safer stem cell therapies, it is critically important to improve the understanding of stem cell regulation. Most of the knowledge about stem cell regulation comes from studies that investigate the aggregate behaviors of thousands to millions of stem cells. However, recent studies suggest that individual hematopoietic stem cells (HSCs) behave substantially differently from one another in both mice and humans. These newly discovered inter-cellular differences present exciting new opportunities for studying HSC regulation. However, they also present significant technical challenges that are difficult to address with conventional approaches. The proposed research program will use a novel systems biology approach combined with quantitative single-cell analysis to determine the cellular and molecular mechanisms underlying HSC heterogeneity and coordination, as well as their influences on aging and the pathogenesis of hematopoietic diseases. Many hematopoietic diseases?such as bone marrow failure, myeloproliferative disorders, myelodysplastic syndromes and other age-associated hematopoietic problems?are initiated by rare cells. The proposed research program will map the precise pathogenesis of these diseases and identify disease founder clones and genes using newly developed single cell tracking and molecular profiling technologies. With these technical advances, the proposed research will also produce new fundamental knowledge about blood regeneration, the foundation of bone marrow transplantation. The long-term objective is to apply the knowledge obtained from the proposed basic research to control blood regeneration for therapeutic use, to detect leukemia and other clonal diseases at early stages, and to effectively treat diseases with advanced stem cell therapies.